(1) Field of the invention
The invention relates to a receiver, more specifically to a radio or TV receiver arranged for receiving broadcast signals. These signals are transmitted by broadcast transmitters and are formed by a carrier signal having a predetermined nominal frequency, on which carrier signal an information signal (AM or FM) is modulated. This information signal is alternatively denoted the "programme". This receiver further comprises one or more preselection keys by means of which the receiver is directly tuneable to a broadcast transmitter assigned to such a key, for receiving the programme transmitted by this transmitter. Assigning a broadcast transmitter to a preselection key, that is to say "programming" the preselection keys, is automated and effected by a control circuit in the form of a micro-computer, on receipt of a preprogramming actionstart-command.
(2) Description of the prior art
As is generally known, broadcast receivers comprise a mixing circuit to which on the one hand an aerial signal supplied by an aerial is applied and on the other hand an oscillator signal. This mixing circuit produces an output signal denoted the intermediate frequency signal. The oscillator signal is generated by a generator of a type in which the frequency of the oscillator signal is determined by a so-called tuning value. This tuning value thus characterizes a specific broadcast transmitter and is a number (usually in binary representation) which is characteristic of the value of a quantity which can be varied in the generator. The actual type of quantity depends on the construction of the generator.
With a present very current embodiment this generator comprises a resonant circuit having a capacitor in the form of a varicap diode. The tuning frequency of this resonant circuit is determined by the capacitance of this capacitor, which capacity itself is determined by a setting voltage applied to the varicap diode. This setting voltage is produced by a digital-to-analog converter and is proportional to the tuning value.
In another, very current embodiment the generator comprises a synthesizer circuit which, as is known, is provided by a phase-locked loop (PLL) and an adjustable frequency divider included in this loop. The frequency of the oscillator signal produced by this synthesizer circuit now depends on the dividing factor of the frequency divider and this dividing factor is proportional to the tuning value.
In order to increase ease of operation, contemporary receivers are often provided with a preselection memory having a plurality, for example N, addressable memory locations (see for example, German Offenlegungsschrift No. 28 50 733). In each memory location the tuning value of a receivable broadcast transmitter is stored. In addition, a control field is provided having a plurality of preselection keys. One of the memory location is unambiguously associated with each key, or with combinations of keys. This means that if a given key or a given combination of keys is operated, the tuning value from a given memory location is applied to the generator.
Preprogramming the preselection memory, that is to say storing tuning values in memory locations, must then indeed be effected manually and separately for each memory location and consequently for each receivable broadcast transmitter. For receivers which are normally speaking not meant to be moved this is usually no problem. For mobile receivers, such as car-radios this will often result in unsurmountable problems, as these receivers will namely be often moved to areas where not all the preprogrammed broadcast transmitters, or even none of them, are receivable, while moreover the associated tuning values of the broadcast receivers which can be received in that area are not known. In practice it has been found that the increase in the ease of operation which is the object of preprogramming the preselection memory can often not be realized. It has therefore already been proposed to have the preprogramming of the preselection memory be effected automatically. A known receiver of this type comprises for that purpose start-command generation means in the form of what is commonly referred to as an ASPM-key (ASPM=Automatic Station Programme Memory) provided on the control field and which, when operated, produces a preprogramming action-start-command. In addition, this known receiver comprises a control circuit in the form of a micro-computer, which starts a preprogramming action in response to the preprogramming action-start-command. To that end it starts generating a fixed sequence of auxiliary tuning values which are sequentially applied to the generator. By means of a detector circuit coupled to the output of the mixing circuit it is determined whether a generated auxiliary tuning value corresponds to a receivable broadcast transmitter. If so, then this auxiliary tuning value is stored in a memory location assigned for that purpose in the preselection memory. Independent of the fact whether the generated auxiliary tuning value corresponds or does not correspond to a receivable broadcast transmitter a further auxiliary tuning value is thereafter generated from the fixed sequence. If all the memory locations have been filled in this way, or when the last auxiliary tuning value of the fixed sequence has been generated, the generation of further auxiliary tuning values is stopped.
It should be noted that the fixed sequence of tuning values has been chosen such that it accurately covers a whole wave-range, for example the overall FM-band.